Selective metal etching in metal/polymer structures

Abstract

A differential material removal process wherein a selected material can be rapidly removed without adverse impact to surrounding layers of different materials. Ultraviolet radiation is used to selectively remove metal without adversely harming adjacent polymer layers, in a metal-polymer multilayer structure. The wavelength (100-400 nm) of the ultraviolet radiation and the energy fluence per pulse are selected so that the removal rate of metal due to thermal processes is significantly greater than the removal rate of the polymer by ablative photodecomposition. This can occur at an energy fluence per pulse level greater than that at which the etch rate of the polymer begins to level off. For example, copper of a thickness less than 5 microns is rapidly etched in one or two pulses while adjacent polyimide layers are substantially unetched by the application of ultraviolet pulses of wave-lengths 248-351 nm, at energy fluences per pulse in excess of approximately 3 or 4 J/cm 2 .

We claim: 
     
       1. A method for removing a metal layer from a structure, in the presence of a polymer layer, without substantial removal of said polymer layer, by the step of irradiating said metal layer with pulsed ultraviolet radiation of a wavelength in the range 100-400 nm and an energy fluence per pulse sufficiently high that said metal is rapidly removed in one or two pulses while only a thin surface region less than a few microns of said polymer is etched by said ultraviolet radiation pulses said energy fluence per pulse being greater than 3-5 J/cm 2 .   
     
     
       2. The method of claim 1, where said metal layer is comprised of copper, and said polymer layer is comprised of polyimide. 
     
     
       3. The method of claim 1, where said structure is a multilayer structure having a polymer layer separating two metal layers. 
     
     
       4. The method of claim 1, where said polymer layer is comprised of materials selected from the group consisting of resists, PMMA, polyimides, and polyesters. 
     
     
       5. The method of claim 1, where said metal layer is comprised of copper and said polymer layer is comprised of polyimide, said wavelengths being in the range 248-351 nm and said energy fluence per pulse being greater than approximately 3J/cm 2 . 
     
     
       6. The method of claim 5, where said structure is a multilayer structure having a copper layer of several microns thickness located on a polyimide layer of several microns thickness. 
     
     
       7. The method of claim 1, where said metal layer is comprised of copper and said polymer layer is comprised of polyimide, said wavelength being 193 nm. 
     
     
       8. A method for selectively removing a metal layer in a structure comprised of a plurality of metal and polymer layers, comprising the steps of: positioning said structure in the path of ultraviolet laser pulses, said structure including at least one layer of a metal and one layer of a polymer, irradiating said structure with ultraviolet laser pulses to remove said metal layer without substantial etching of said polymer layer,   said ultraviolet laser pulses having a wavelength and energy fluence per pulse such that the rate of removal of said metal layer by said laser pulses is at least twice as rapid as the rate of removal of the polymer layer by said laser pulses, for each said laser pulse.   
     
     
       9. The method of claim 8, where said metal layer is comprised of copper and said polymer layer is comprised of polyimide. 
     
     
       10. The method of claim 8, where said wavelength is in the range 100-400 nm and said energy fluence per pulse is greater than 3-5 J/cm 2 . 
     
     
       11. The method of claim 8, where said wavelength and energy fluence per pulse are chosen such that said polymer has high absorption for said ultraviolet radiation and is ablatively photodecomposed to only a small depth less than a few microns, while said metal layer is completely removed. 
     
     
       12. The method of claim 8, where the wavelength of said ultraviolet radiation is 248 nm and the energy fluence per pulse of said ultraviolet radiation is sufficiently high that the curve of etch depth/pulse versus energy fluence/pulse exhibits a leveling-off. 
     
     
       13. The method of claim 8, where the wavelength of said ultraviolet radiation is 308 nm, and the energy fluence per pulse of said radiation is sufficiently high that the curve of etch depth/pulse versus energy fluence/pulse of said radiation has begun to level off. 
     
     
       14. The method of claim 8, where the wavelength of said ultraviolet radiation is 351 nm, and the energy fluence per pulse of said radiation is sufficiently high that the curve of etch depth/pulse versus energy fluence/pulse of said radiation has begun to level off. 
     
     
       15. A method for selectively etching a metal layer in the presence of a polymer layer, where said metal layer has a thickness of several microns and said polymer layer also has a thickness of several microns, said polymer being comprised of a material which exhibits an etch depth/pulse versus energy fluence/pulse curve which begins to level off with increasing energy fluence beyond a fluence F=F 0 , when said polymer is ablatively photodecomposed by ultraviolet laser pulses having a wavelength in the range 100-400 nm, said removal process including the step of applying a single pulse of said ultraviolet radiation, said pulse having an energy fluence sufficiently high to completely remove said metal layer, said fluence being greater than F 0 .   
     
     
       16. The method of claim 15, where said ultraviolet radiation pulse has a wavelength and energy fluence such that said metal layer is removed by a thermal mechanism and said polymer layer is only slightly removed by an ablative photodecomposition mechanism. 
     
     
       17. The method of claim 16, where said metal layer is comprised of copper and said polymer layer is comprised of polyimide. 
     
     
       18. A method for removing a copper layer located adjacent to a polyimide layer without substantial removal of said polyimide layer, said method including the steps of applying a number of pulses ≦5 of ultraviolet radiation to said copper layer, said radiation having a wavelength in the range 100-400 nm and an energy fluence/pulse greater than about 3-5J/cm 2 . 
     
     
       19. The method of claim 18, where said wavelength is 248 nm. 
     
     
       20. The method of claim 18, where said wavelength is 308 nm. 
     
     
       21. The method of claim 18, where said wavelength is 351 nm.